Ships are designed so that when loaded they can be sank enough to secure an appropriate draft and stabilized, with their propeller screw also under water. Accordingly when not loaded, ships are floated up by buoyancy too much to be stabilized or to secure a submersing depth of the propeller screw. Unloaded cargo ships and the like therefore take in the seawater at the port of call and store the seawater in the hull so as to have a height of the waterline close to that in a loaded state. The seawater taken in as above is called ballast water.
Ballast water is transported to the next port of call as a “weight” of the ship, and released when cargoes are loaded. In other words, marine organisms in the previous port of call can be brought to the next port of call. Transplanting organisms from one place to another place is likely to result in destruction or contamination of ecosystems grown by Mother Nature in the latter place. Therefore the International Maritime Organization (IMO) has adopted the Ballast Water Convention (International Convention for the Control and Management of Ships' Ballast Water and Sediments) which defines standards for an amount of organisms included in ballast water to be discharged.
According to the standards, a number of organisms of 50 μm or greater (mostly zooplankton) included in 1 m3 of ballast water discharged from a ship must be less than 10. The number of organisms of greater than or equal to 10 μm and smaller than 50 μm (mostly phytoplankton) in 1 mL must be less than 10. The number of cholera bacilli in 10 mL must be less than 1 cfu. The number of Escherichia coli in 100 mL must be less than 250 cfu. The number of Enterococcus in 100 mL must be less than 100 cfu. Here, “cfu” stands for colony forming unit.
To meet the standards, microorganisms in the seawater taken in as the ballast water need to be killed. Methods for killing microorganisms in the seawater include a method for physically or mechanically killing aquatic organisms, a method for thermally killing aquatic organisms, and a method for injecting chemicals into the ballast tank or generating chlorine-based substances and the like to kill aquatic organisms.
A method for applying a voltage between electrodes to pass an electric current and electrolyze seawater to generate sodium hypochlorite has the advantages that no disinfectant needs to be supplied and the device itself can be reduced in size. Patent Literature 1 discloses a device for killing marine microorganisms which kills marine microorganisms by using physical killing power from electrical shocks in addition to chemical sterilizing power of sodium hypochlorite generated by the electrolysis of the seawater flown from one end of cylindrical electrodes.
Here, one of the electrodes (cathode side) has through holes. The other electrode is formed with a cylindrical outer surface with no hole. Generation points of chlorine gas (or hypochlorous acid) are provided on the other electrode (anode side) opposed to the holes.